Core network is the segment that connects the RAN with operators' in-house application servers, IP multimedia subsystem or the Internet. It is comprised of both circuit switched and packet switched elements. Historically, it supported only circuit switched (CS) services, but later with the advent of 3G it started to support packet switched (PS) services as well. LTE and 4G systems only require PS support and with the passage of time, core networks are expected to only provide for IP/Ethernet services.